Stagger, in the case of Air Canada vs. Air Transat, the latter have 'Type A' exits at the Door 3 position on their A330-300s. This allows a higher rate of exit than the 'Type 1' emergency exits in the same position used on Air Canada's 330-300s.
Consequently, the aircraft is licensed for a higher passenger number in the rear cabin. The same licensing arrangements forced Airbus to add an overwing exit to the A340-600, as 4 Type A doors was not thought sufficient in an aircraft of that size.